(Reuters) - Poshmark, an online service that helps people buy and sell second-hand clothing, is in talks to raise more than $50 million in a funding round that could value the startup at roughly $600 million, people familiar with the matter said on Monday.

The round will test investor appetite for a new wave of e-commerce companies that replace traditional second-hand stores and thrift shops. The sector has become competitive with rivals such as ThredUp, Tradesy and The RealReal fighting for share with Poshmark in the online used-clothing market.

Poshmark, which is working with investment bank Morgan Stanley, has begun negotiations with growth equity investors, the four people said, asking not to be named because the matter is private. There is no guarantee Poshmark will receive funding at the $600 million valuation it is seeking, the people said.

Poshmark could not immediately be reached for comment. Morgan Stanley declined to comment.

Poshmark allows people interested in selling clothing and accessories to post photos and details on the app, allowing other Poshmark users to browse through items. When an item sells, the seller receives a pre-paid shipping envelope and Poshmark gets a 20 percent cut of every sale.

Poshmark's Chief Executive and co-founder Manish Chandra also started a social network shopping website Kaboodle, which was acquired by Hearst Media in 2007.

The website boasts 25 million items uploaded for sale on its website, including such high-end brands as Gucci, Tiffany & Co and Louis Vuitton. About $4 million worth of inventory is uploaded into the marketplace every day, Poshmark said on its website.